Surface Chemistry for JEE Main 2014 Preparation


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Surface Chemistry for JEE Main 2014 Preparation, How to Learn Chemistry?, JEE Main 2014 Preparation Tips

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Surface Chemistry for JEE Main 2014 Preparation

  1. 1. Surface Chemistry “The branch of physical chemistry, which deals the nature of surfaces and also with the chemical and physical processes which takes place on the surfaces, is called surface chemistry”. In surface chemistry, we study the phenomenon of adsorption, catalysis and colloidal properties. Adsorption (1) Definition: The phenomenon of attracting and retaining the molecules of a substance on the surface of a liquid or solid resulting in to higher concentration of the molecules on the surface is called adsorption.
  2. 2. (2) Causes of adsorption: Unbalanced forces of attraction or free valencies which is present at the solid or liquid surface, have the property to attract and retain the molecules of a gas or a dissolved substance on to their surfaces with which they come in contact. Example: Ammonia gas placed in contact with charcoal gets adsorbed on the charcoal whereas ammonia gas placed in contact with water gets absorbed into water, (3) Difference between adsorption and absorption: Adsorption Absorption It is a surface It concerns with the
  3. 3. phenomenon. whole mass of the absorbent. In it, the substance is It implies that a only retained on the substance is uniformly surface and does not distributed, through the go into the bulk or body of the solid or interior of the solid or liquid. liquid. In it the concentration In it the concentration of the adsorbed is low. molecules is always greater at the free phase.
  4. 4. It is rapid in the It occurs at the uniform beginning and slows rate. down near the equilibrium. Examples : (i) Water Examples : (i) Water vapours adsorbed by vapours absorbed by silica gel. anhydrous CaCl2 (ii) NH3 is adsorbed by (ii) NH3 is absorbed in charcoal. water forming NH4OH (4) Surface forces: Only the surface atoms of an adsorbent play an active role in adsorption. These atoms posses
  5. 5. unbalanced forces of various types such as, Vander Waal‟s forces and chemical bond forces. Thus, the residual force-field on a free surface which is responsible for adsorption is produced. For example, when a solid substance is broken into two pieces, two new surfaces are formed and therefore, the number of unbalanced forces becomes more. As a result the tendency for adsorption becomes large. (5) Reversible and Irreversible adsorption: The adsorption is reversible, if the adsorbate can be easily removed from the surface of the adsorbent by physical methods. If the adsorbate can not be easily removed from the
  6. 6. surface of the adsorbent is called irreversible adsorption. (6) Characteristics of adsorption: (i) Adsorption refers to the existence of a higher concentration of any particular component at the surface of a liquid or a solid phase. (ii) Adsorption is accompanied by decrease in the ΔG (free energy change) of the system when ΔG = 0, adsorption equilibrium is said to be established. (iii) Adsorption is invariably accompanied by evolution of heat, i.e. it is an exothermic process. In other words, ΔH of adsorption is always negative.
  7. 7. (iv) When a gas is adsorbed, the freedom of movement of its molecules becomes restricted. On account of it decrease in the entropy of the gas after adsorption, i.e. ΔS is negative. (v) For a process to be spontaneous, the thermodynamic requirement is that ΔG must be negative, i.e. there is decrease in free energy. On the basis of Gibb‟s Helmholtz equation, ΔG = ΔH – T ΔS, ΔG can be negative if ΔH has sufficiently high negative value and T ΔS has positive value.
  8. 8. Classification of adsorption Adsorption can be classified into two categories as described below, (1) Depending upon the concentration: In adsorption the concentration of one substance is different at the surface of the other substance as compared to adjoining bulk or interior phase. (i) Positive adsorption: If the concentration of adsorbate is more on the surface as compared to its concentration in the bulk phase then it is called positive adsorption.
  9. 9. Example: When a concentrated solution of KCl is shaken with blood charcoal, it shows positive adsorption. (ii) Negative adsorption: If the concentration of the adsorbate is less than its concentration in the bulk then it is called negative adsorption. Example : When a dilute solution of KCl is shaken with blood charcoal, it shows negative adsorption. (2) Depending upon the nature of force existing between adsorbate molecule and adsorbent (i) Physical adsorption : If the forces of attraction existing between adsorbate and adsorbent are
  10. 10. Vander Waal‟s forces, the adsorption is called physical adsorption. This type of adsorption is also known as Physisorption or Vander Waal‟s adsorption. It can be easily reversed by heating or decreasing the pressure. (ii) Chemical adsorption : If the forces of attraction existing between adsorbate particles and adsorbent are almost of the same strength as chemical bonds, the adsorption is called chemical adsorption. This type of adsorption is also called as Chemisorption or Langmuir adsorption. This type of adsorption cannot be easily reversed.
  11. 11. Factors which affect the extent of adsorption The following are the factors which affect the adsorption, (1) Nature of the adsorbate (gas) and adsorbent (solid) (i) In general, easily liquefiable gases e.g., CO2, NH3, Cl2 and SO2 etc. are adsorbed to a greater extent than the elemental gases e.g. H2, O2, N2, He etc. (while Chemisorption is specific in nature.) (ii) Porous and finely powdered solid e.g. charcoal, fullers earth, adsorb more as compared to the hard non-porous materials. Due to this property powdered charcoal is used in gas masks. (2) Surface area of the solid adsorbent
  12. 12. (i) The extent of adsorption depends directly upon the surface area of the adsorbent, i.e. larger the surface area of the adsorbent, greater is the extent of adsorption. (ii) Surface area of a powdered solid adsorbent depends upon its particle size. Smaller the particle size, greater is its surface area. (3) Effect of pressure on the adsorbate gas (i) An increase in the pressure of the adsorbate gas increases the extent of adsorption.
  13. 13. (ii) At low temperature, the extent of adsorption increases rapidly with pressure. (iii) Small range of pressure, the extent of adsorption is found to be directly proportional to the pressure. (iv) At high pressure (closer to the saturation vapour pressure of the gas), the adsorption tends to achieve a limiting value. (4) Effect of temperature (i) As adsorption is accompanied by evolution of heat, so according to the Le-Chatelier‟s principle, the magnitude of adsorption should decrease with rise in temperature.
  14. 14. (ii) The relationship between the extent of adsorption and temperature at any constant pressure is called adsorption isobar. (iii) A physical adsorption isobar shows a decrease in x/m (where „m‟ is the mass of the adsorbent and „x‟ that of adsorbate) as the temperature rises. (iv) The isobar of Chemisorption show an increase in the beginning and then decrease as the temperature rises. Please visit for more information.
  15. 15. Send us your queries at - Team Ednexa